It is often desirable to know the constituents or substances in a gaseous cloud from a distance before any personnel enter or venture near to the gaseous cloud. In particular, in modern times many chemical and biological agents, such as those for chemical and biological warfare, can be emitted into the atmosphere as a gaseous or amorphous cloud. When these substances are emitted into the atmosphere it is generally unknown what substances are present and what precautions must be taken. It is also desirable to know, also from a standoff distance, what substances might be present in a chemical plume such as from a chemical fire or spill. Though these are merely exemplary, it is understood that it is generally desirable to know the constituents and substances present in a gaseous cloud or region before any individual comes in contact with, or close proximity to, the cloud or region.
Though lab techniques for identifying different compounds and biological agents are generally well known in the art, doing such identification at a distance without confining the substance in any manner is often very difficult. Many times chemical spills, accidents, or even intentional acts, which release harmful chemical and biological agents into the atmosphere, are done well outside of controlled circumstances that allow for easy testing and identification of the chemical agents in the plume. It is desirable to identify unknown substances so that the personnel who must approach the plumes can take appropriate precautions.
One method uses raman back scattering spectroscopy. In this method, a monochromatic laser is pulsed through a cloud or region of suspicious gas. The internal vibrational characteristics of the molecules back scatter some of the photons without changing them if the photons match the vibrational characteristics of the molecules. The signals from a raman back scattering spectroscopy are generally very weak and therefore require that a very powerful laser be used for this purpose.
Other techniques such as Differential Absorption Lidar (DIAL) use infrared wavelengths to identify chemical species in a manner similar to conventional infrared absorption spectroscopy. An infrared laser beam is shined at a cloud or region of gas and light is reflected from the cloud. As the light is scattered back, different conditions act on the reflected light, such as range absorption of the suspicious region and atmosphere scattering, all allowing for the characterization of absorption lines to characterize the various chemical species present in the cloud or plume.
These systems, however, have inherent drawbacks such as requiring high powered lasers with very specific wavelength capabilities and highly absorbable infrared radiation. Many techniques that are generally used in the lab provide for highly specific characterizations of unknown molecules which these previously known methods and systems do not provide. One known technique can identify unknown gaseous molecules and even quantify the same photo acoustic spectroscopy (PAS). With reference to FIG. 1, a well-known PAS apparatus 10 is shown. The PAS apparatus 10 can be used to identify and quantify a gaseous substance which has been placed in a sample cell 12.
Briefly, PAS apparatus 10 works by providing a black body 14 which emits a ray 16 of infrared (IR) light. This IR light is focused by a mirror 18 towards the sample cell 12. A tuning source 19, such as a diffraction grading, limits the frequency of infrared light reaching the sample cell 12 at any given moment. Therefore, the frequency of infrared light reaching the sample cell 12 is known. The tuning apparatus 19 can tune the infrared ray of light 16 over a plurality of frequencies. A chopper 20 chops the ray of light 16 before it enters the sample cell 12. This produces an intermittent ray 16a of light. The sample cell 12 includes a gaseous material which, when heated by a specific frequency of the ray 16a of IR light, expands. Therefore, when the ray of light 16 is tuned to a specific frequency it will cause an expansion of the sample in the sample cell 12, if the sample absorbs that specific frequency of light. If the sample in the sample cell 12 expands, it will also then contract as that frequency of light is changed. This expansion and contraction of the sample cell produces a photo acoustic effect.
Microphones 22 and 24 detect the photo acoustic effect produced in the sample cell 12 and send the response to a processor 26. The processor 26 can then determine the identify of the sample that is present in the sample cell 12. The processor 26 knows the frequency of the ray of light 16 that reach the sample cell 12 to produce the photo acoustic effect at any given moment. Therefore, as each of a plurality of frequencies of light reach the sample cell 12, the processor 26 can create a spectrum for the sample in the sample cell 12. Comparing the spectrum to known spectrums, the processor 26 can then determine the identity of the sample in the sample cell 12. Quantification of the sample may also be performed due to the fact that the greater concentration of sample particles the greater the photo acoustic effect detected by the microphones 22, 24.
It would be highly desirable to be able to use such a system from a standoff distance to determine the substances in a gaseous cloud or region without enclosing that cloud or region in a sample cell. This would allow for determination of a gaseous substance from a distance without requiring high powered lasers or other devices.